Exploitation of renewable energy, such as solar energy and the development of improved solar technology are of great interest in today's energy hungry world. Currently, industrial solar cells typically yield only 17% efficiency. The level of efficiency may be due reflection of light from the cell surface, due to the inherently high refractive index of silicon. The loss due to reflection can reach more than 40-45%, resulting in a low photoelectric conversion efficiency. Solar cells composed of silicon with no treatment of the light absorptive surface reflect approximately ⅓ of the incident light. This light is therefore wasted in the production of electricity from the solar cell.
It would therefore be desirable to make silicon wafers or substrates used in most solar cells more light absorptive to increase energy conversion efficiency. Black silicon refers to silicon surfaces or silicon-based films (including surface or film of silicon compound) that have high light absorptive surfaces. Compared to normal untreated silicon material the goal of black silicon is to have strong light absorption properties. It has been found that if black silicon is applied to optical sensor or solar cell the reflection of light from the cell surface can be reduced below 5%, often below 2%, increasing the efficiency of light sensing and absorption, and the conversion efficiency of solar cell can be markedly improved.
Prior teachings for producing low reflectance silicon have involved complex and often expensive manufacturing techniques and/or equipment. In some methods, the reflection reduction is not uniform across the spectrum of light incident on the solar cell. Some methods require multiple steps and multiple solutions. Further, some nanoscale texturing requires very precise etching. If the etching is not sufficient or precise, then the black silicon effect will not be realized and the reflectivity of the silicon surface will remain too high for practical use. Contrastingly, over-etching leads to (a) increased surface defects; and (b) the possibility of destroying existing structures on the surface of the silicon such as emitter junctions. Accordingly, the etching must be very uniform to ensure that all areas of the wafer are etched to the same extent, otherwise a silicon wafer or substrate may contain areas that are over- and/or under-etched. This requirement becomes even more challenging when the uniformity must occur over larger wafers such as the 125 mm or 156 mm square wafers used in current solar cell manufacturing.
Nanoscale structure etching processes are very sensitive to local fluid flows during the etching process. Further, it has been found that natural variations in flow caused by convection and substrate edges lead to an unacceptable variation in etching across, such as with a typical 125 mm silicon wafer. Some methods utilize a large volume of fluid with the substrate completely immersed in the fluid. These methods may result in poor fluid flow, concentration build ups, and/or other local fluid flow problems because the fluid may not flow evenly between the substrates and over their surfaces. These “bath” or “tank” processes are wasteful because they necessarily provide extra fluid beyond that which is in direct in contact with the wafer. Further, such methods are imprecise because of the improper flow, micro-currents within the baths, improper mixing, settling of reagents, etc. Such processes inherently cause non-uniformity. While intentional mixing of the volume to counteract some of the abovementioned effects can be accomplished with mechanical means such as stirring, substrate movement, or sonication, these methods cannot ensure uniform mixing across an entire wafer.
Accordingly, a need exists for a manufacturing process for etching silicon wafers that is precise, easy to implement in a manufacturing environment, provides controllability and uniformity in etching across the surface of the wafer, and does not require and waste large quantities of reagent fluid. The present disclosure addresses and overcomes the hereto prior problems.